A Suggested Method
(If you are a beginner, start with middle box on the top row.)

Always the next box you try to fill is the diagonal (right, up) box.
Sometimes this may not be possible. Then do one of the following.

1.

The box is already filled:
Fill the box just below.

2.

The box 'falls' above the square:
Fill the box vertically below, on the bottom row.

3.

The box 'falls' to the right of the square:
Fill the box horizontally to the left, on the left most column.

Examples:

1 is filled in middle box, top row.
2 goes above the square; it is filled on the bottom row.
3 is the normal diagonal right, up box.
4 goes to the right of the square; it is filled on the left most column.
5 is the normal diagonal right, up box.
6 is the 'already filled box' case; so it is filled just below, and so on.

This method can be used to fill numbers in any 'odd x odd' magic square.
Try it out.